question_answer 12)

 FeSO4 solution mixed with (NH4)2SO4 solution in the molar ratio 1 : 1 gives the test of Fe2+ ion but CuSO4 solution mixed with aqueous NH3 in the molar ratio of 1 : 4 does not give the test of Cu2+. Explain why.

Answer:

FeSO4 does not form any complex with (NH4)SO4. Instead, they form a double salt, FeSO4.(NH4)2SO4.6H2O (Mohr salt) which dissociates completed in ion in the solution. Hence, it gives the test of Fe2+ ions. CuSO4 solution mixed with aqueous ammonia in 1 : 4 molar ratio forms the complex with the formula [Cu(NH3)]SO4 in which the complex ion, [Cu(NH3)4]2+ does not dissociate to give Cu2+ ions. Hence, it does not give the tests of Cu2+ ion.